Formic acid : properties

formic acid

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Chemical properties of formic acid :

HCOOH is the strongest acid among all the members of homologous series.

1. Acidic Nature:

a) It is monobasic acid. Its acidic properties are due to its ionization in aqueous solution.

HCOOH —> HCOO^– + H⁺

b) It reacts with metal carbonates and bicarbonates and evolve CO₂.

HCOOH + NaHCO₃ ——-> HCOONa +H₂O + CO₂

2 HCOOH + Na₂CO₃ ——-> 2 HCOONa + 2 H₂O + CO₂

c) It reacts with alkalies to form corresponding salts.

HCOOH + NaOH ——-> HCOONa +H₂O

HCOOH +NH₄OH ——-> HCOONH₄ +H₂O

d) Highly electropositive metals evolve H₂ when react with HCOOH.

2 HCOOH + 2 Na ——–> 2 HCOONa + H₂

e) It combines with alcohol to form ester.

HCOOH + CH₃OH —–> HCOOCH₃ (Methyl formate) + H₂O

f) It reacts with PCl₅ or SOCl₂ to give formyl chloride which is not a stable compound. It decomposes into Hydrochloric acid  &  Carbon monoxide.

HCOOH + PCl₅ ——-> HCOCl + POCl₃ +HCl

HCOCl ——> HCl+ CO

HCOOH + SOCl2 ——-> HCOCl + SO2+HCl

HCOCl ——> HCl+ CO

2. Action of Heat :

when heated above 160 ⁰ C, it decomposes to give  Carbon dioxide and Hydrogen.

HCOOH ——-> CO₂+ H₂

3. Dehydration :

when formic acid is warmed with concentrated H₂SO₄. It decomposes to give Carbon monoxide & water.

HCOOH ——–> CO + H₂O

4. Action of heat on formates :

a) When Na-formate is heated to 36O ⁰C, it decomposes to form Sodium oxalate and Hydrogen .

2 HCOONa ——> (COONa)2  (sodium oxalate) + H₂

b) When sodium-formate is heated with soda lime then Hydrogen gas is formed.

HCOONa +NaOH (CaO) ——-> H₂ + Na₂CO₃

c) Formaldehyde  is formed when dry Ca-formate is heated.

d) Formamide is formed when ammonia formate is heated.

HCOONH₄ ——-> HCONH₂ + H₂O

5) Reducing Properties:

Like aldehydes, formic acid behaves as reducing agent it is oxidized to  carbonic acid (unstable acid ), which decomposes into Carbon dioxide  and water.

HCOOH——> H₂CO₃ (unstable acid) ——> CO₂ +H₂O

a) It decolorizes acidified KMnO4 solution :

2KMnO₄ + 3H₂SO₄ ——> K₂SO₄ + 2MnSO₄ + 3H₂O + 5(O)

HCOOH + O ——–> CO₂ + H₂O ]x 5

5 HCOOH + 2KMnO₄ + 3H₂SO₄    ——> K₂SO₄ + 2MnSO₄ + 8H₂O +5CO2

b) It reduces mercuric chloride ( HgCl₂ ) to white precipitate of mercurous chloride ( Hg₂Cl₂) and then grey precipitate of mercury (Hg) :

HCOOH + 2HgCl₂ ——-> Hg₂Cl₂ + CO₂ + 2HCl

2 HCOOH +Hg₂Cl₂ ——> 2Hg + 2CO₂ + 2HCl

c) It reduces ammonical  AgNO3  (Tollen’s reagent)

HCOOH + Ag₂O ——> CO₂ + H₂O + 2Ag

d) It reduces Fehling solution i.e gives red precipitate of  cuprous oxide (Cu₂O) :

HCOOH + 2CuO —-> Cu₂O( red ppt of cuprous oxide) + H₂O+ CO₂